For all semiconductor lasers it is very advantageous to confine the carrier injection as much as possible to the active area that contributes to the lasing activity. Carriers that are injected at spatial positions without overlap with the mode profile are lost. Thus, the better the carrier confinement, the higher is the overall efficiency.
A known measure for confinement of the current close to the center of a VCSEL is an aperture layer having a higher conductivity in the center of the cavity and a lower conductivity at its edge. An aperture layer can be realized by lateral oxidation of a high aluminum containing layer or by local proton implantation. However, the carrier injection for both approaches (oxidation and implantation) is not necessarily adapted to the lateral intensity profile of the optical mode.